1. Field of the Invention
The present invention relates to charge transport layers which contain a copolycarbonate resin with a specified molecular structure that imparts improved durability over a broad range of operating temperatures. The present invention also relates to organic photoconductive imaging receptors which contain such a charge transport layer.
2. Discussion of the Background
A general discussion of electrophotography (photocopying) is given in Kirk-Othmer, Encyclopedia of Chemical Technology,4th ed, vol. 9, pp. 245-277, Wiley, New York (1994), and a brief description of laser beam printing is provided in Encyclopedia of Electronics, 2nd ed, Gibilisco et al, Eds., pp. 669-671, TAB BOOKS, Blue Ridge Summit, Pa. (1990), both of which are incorporated herein by reference.
Photoreceptors are the central device in photocopiers and laser beam printers. In most photocopiers and laser beam printers, the photoreceptor surface is contained on the outside surface of a hollow metal cylinder, called a drum. Typically, the drum is made of a metal, such as aluminum, which may be anodized or coated with a thin dielectric layer (injection barrier) which is in turn over coated with photogeneration and photoconduction layers.
Key steps in transfer electrophotography include the charging step, the exposure step, the development step, and the transfer step. In the charging step, gas ions are deposited on the surface of the photoconductor drum. In the exposure step, light strikes the charged photoreceptor surface and the surface charges are neutralized by mobile carriers formed within the photoreceptor layer. Thus, the charge on the surface is transmitted in the exposed areas of the photoconductive layer to the oppositely charged metal substrate of the drum. In the development step, a thermoplastic pigmented powder (toner) which carries a charge is brought close to the photoreceptor so that toner particles are directed to the charged image regions on the photoreceptor. In the transfer step, a sheet of paper is brought into physical contact with the toned photoreceptor and the toner is transferred to the paper by applying a charge to the back side of the paper.
Presently, the most suitable photoconductive imaging receptors for low and medium speed electrophotographic plain-paper copiers and laser printers have a double-layered configuration. Photogeneration of charge carriers (electron-hole pairs) takes place in a thin charge generation layer (CGL), typically 0.5 .mu.m thick, which is coated on a conductive substrate such as an aluminum drum. After photogeneration, mobile carriers (usually holes) are injected into a thicker charge transport layer (CTL), which is about 21 .mu.m thick and coated on top of the CGL, under an electric field gradient provided by a negative surface charge. These holes drift to the outermost layer of the photoreceptor to selectively neutralize surface charger thereby forming a latent electrostatic image, which is subsequently developed by thermoplastic toner.
The physical durability of the organic photoconductive imaging receptor is the major characteristic that determines service lifetime, and such durability depends on the mechanical properties of the surface CTL. The CTL is formulated from two major components. They are electron-donor molecules responsible for hole transport, known as the charge-transport material (CTM), and an appropriate binder resin, which must be amorphous and transparent to light. The CTM is usually a low molecular weight organic compound with arylamine or hydrazone groups, and it is selected primarily on the basis of solubility, compatibility with the binder resin, charge transport property, and electrophotographic cyclic stability. The CTM is a non-reactive binder resin diluent (molecular dopant), and it must be compatible in approximately equal parts by weight with the binder resin to ensure good charge mobility, which involves electron hopping between adjacent molecules of the CTM.
The role of the binder resin is to impart the physical durability necessary for acceptable lifetime under the service conditions encountered in copiers and printers. It is well known that the most suitable binder resins belong to the general class of aromatic polycarbonates (PCR), which exhibit such desirable characteristics as solubility (to allow film coating from solution), high carrier mobility, compatibility with the CTM, transparency, durability, adhesion to the CGL, and so on. The simplest and best known example is bisphenol-A polycarbonate (BPA-PCR), more formally called poly[2,2-bis-(4-phenylene)propane carbonate], which has good impact strength and toughness.
However, these attributes of BPA-PCR are degraded by dilution with the CTM. Furthermore, because of its symmetrical structure, BPA-PCR has poor stability in solution, and it also has a tendency to stress crack and phase separate from the charge transport material in the solid state, leading to an opaque charge transport layer with unacceptable performance. Finally, BPA-PCR is not so resistant to surface scratching and abrasion during the copy (print) process, caused by physical contact of the surface of the CTL with paper and machine components designed for the addition and removal of toner.
U.S. Pat. No. Re. 33,724 discloses poly[1,1-bis-(4-phenylene)cyclohexane carbonate], commonly known as BPZ-PCR, a commercial product designated "IUPILON Z" from Mitsubishi Gas Chemical of Japan, as an improved polycarbonate binder resin for organic photoconductive imaging receptors. However, the abrasion resistance properties of organic photoconductive imaging receptors containing BPZ-PCR as a binder in the CTL are not completely satisfactory.
U.S. Pat. No. 5,227,458 discloses polycarbonates obtained from dihydroxydiphenyl cycloalkanes. However, this reference does not disclose any organic photoconductors containing the disclosed polycarbonates. U.S. Pat. No. 5,332,635 discloses electrophotographic photosensitive layers containing a specified polycarbonate.
Abrasion of polymers is a complex phenomenon, involving both surface and bulk properties. Generally, at least two basic kinds of abrasion mechanism are involved: scratch (penetration and plowing of the polymer matrix by a hard asperity); and fatigue (gradual loss of the entire surface layer by repetitive cyclic loading under adhesive contact). Under the actual service conditions of organic photoconductive imaging receptors, there are contributions from several types of abrasion mechanisms, but cyclic fatigue is the major factor. This may be reduced by absorption and dissipation of external stress as internal heat, which can quickly and harmlessly diffuse through the thin CTL into the aluminum substrate, provided there is efficient coupling to a mechanical loss process at the temperature of operation. Otherwise, mechanical stress remains concentrated at the surface, with the likelihood of increased abrasion. It has recently been determined that the resistance of a CTL to mechanical fatigue by cyclic stress correlates with the temperature profile of the dynamic mechanical loss modulus. Two mechanical loss peaks are of significance: 1) the primary relaxation (.alpha. peak), which occurs at a higher temperature and results from long-range segmental motion at the glass-transition temperature, T.sub.g ; and 2) the secondary sub-T.sub.g relaxation (.gamma. peak), which occurs at lower temperature.
However, to date there is no known binder resin which affords a CTL having maximum abrasion resistance, combined with other desirable mechanical performance characteristics such as toughness, impact resistance, and a high heat-distortion temperature. Thus, there remains a need for improved CTL which exhibit increased abrasion resistance and high-temperature rigidity. There also remains a need for organic photoconductive imaging receptors which contain such a CTL.